The present invention relates to apparatus used for the electroerosive working of workpieces. More particularly, the invention relates to packaging of electrical componentry in an integrated gapbox in close relation to the working gap for providing the generation of desired machining pulses to the wire and workpiece. The gapbox of the present invention is relatively simple in construction, and is simple to install while being effective in operation.
Electro-erosive working methods, particularly spark-erosive cutting methods, are largely based on empirical findings and hypotheses, because there has hitherto been no conclusive physical description of the discharge processes in the work gap. Thus, in the case of cutting processes using a wire or strip electrode, apart from the effects of gravity, a problem is also caused by vibrations formed as a result of electromagnetic, electrostatic, as well as mechanical or hydraulic effects.
In view of such disturbances, particularly high demands are made on the power modules, comprising generators and switching circuitry, used to provide the desired machining pulses to the wire and workpiece. The time control of the discharge process takes place by the controlled switching on or off of the generator with the use of a plurality of power switching circuits placed in parallel relation to the wire electrode for generating selectable pulse shapes for the discharge current. For example, a clock may provide clock pulses at a predetermined frequency, and this pulse train is used as a time pattern of trigger pulses to be supplied to the switching circuits as from stored instructions. Desired profiles of pulses may be set as well. In this manner, the erosion process may be readily controlled. Examples of such cutting methods and processes may be found in U.S. Pat. Nos. 3,928,163 and 4,655,888.
Disturbances of the erosion process may occur, however, in the distribution of the discharge signals along signal-carrying media to the workpiece. Transmission line disturbances in the signal-carrying media can create undesirable effects in the machining process. For example, during the transmission of pulses supplied to the wire, inductive effects as well as distributed capacitive effects in the discharge signal leads tend to distort the desired waveform. In addition, irregular switching of the plurality of switching circuits may cause undesirable machining pulses which result in less accurate machining or even damage to the switching circuitry.
It is therefore known to provide diodes at the outputs of the power modules for isolation of the outputs. In these systems, the switching circuitry and isolation diodes normally are placed in a power control cabinet having their outputs connected to cabling leading from a location distant from the working area and working gap. When the erosion pulse becomes active and inactive, the lead length from the outputs of the isolation diodes to the wire electrode cause a transmission line effect wherein collapsing magnetic fields in the wiring create voltage spikes relative to multiples of the length of wire. The p-n junctions of the isolation diodes tend to dissipate energy caused by these transient voltage spikes which reduces the energy applied to the working gap. Accordingly, use of such machining pulse circuitry in known arrangements generates great amounts of heat. Moreover, the componentry is susceptible performance degradation and frequent breakdowns due to this effect.
In addition, secondary inductance and distributed capacitance appears in the cabling arrangements between the output of the isolation diodes and the working area when the diodes are placed in such a remote location notwithstanding the resistive losses in the wire. These effects tend to distort the desired waveform of machining pulses supplied to the working gap and wire.
Prior approaches to this problem provide tuning circuitry for the output lines placed in the power delivering output circuitry. However, where tooling, the type of workpiece, and wire are constantly rearranged depending upon the particular application, such tuning circuitry must constantly be modified. Moreover, such approaches fail to address the transmission line effect caused by collapsing magnetic fields in the erosion process.